Magnetic memories, particularly magnetic random access memories (MRAMs), have drawn increasing interest due to their potential for high read/write speed, excellent endurance, non-volatility and low power consumption during operation. An MRAM can store information utilizing magnetic materials as an information recording medium. One type of MRAM is a spin transfer torque random access memory (STT-RAM). STT-RAM utilizes magnetic junctions written at least in part by a current driven through the magnetic junction. A spin polarized current driven through the magnetic junction exerts a spin torque on the magnetic moments in the magnetic junction. As a result, layer(s) having magnetic moments that are responsive to the spin torque may be switched to a desired state.
For example, FIG. 1 depicts a conventional dual magnetic tunneling junction (MTJ) 10 as it may be used in a conventional STT-RAM. The conventional dual MTJ 10 typically resides on a bottom contact 11, uses conventional seed layer(s) 12 and includes a conventional antiferromagnetic (AFM) layer 14, a conventional first pinned, or reference layer 16, a conventional first tunneling barrier layer 18, a conventional free layer 20, a conventional second tunneling barrier layer 22, a conventional second pinned layer 24, a conventional second AFM layer 26 and a conventional capping layer 28. Also shown is top contact 30.
Conventional contacts 11 and 30 are used in driving the current in a current-perpendicular-to-plane (CPP) direction, or along the z-axis as shown in FIG. 1. The conventional seed layer(s) 12 are typically utilized to aid in the growth of subsequent layers, such as the AFM layer 14, having a desired crystal structure. The conventional tunneling barrier layers 18 and 22 are each nonmagnetic and are, for example, a thin insulator such as MgO.
The conventional pinned layers 16/24 and the conventional free layer 20 are magnetic. The magnetizations 17 and 25 of the conventional reference layers 16 and 24 fixed, or pinned, in a particular direction, typically by an exchange-bias interaction with the AFM layers 14 and 26. Although depicted as a simple (single) layer, the conventional reference layers 16 and 24 may include multiple layers. For example, the conventional reference layers 16 and/or 24 may be a synthetic antiferromagnetic (SAF) layer including magnetic layers antiferromagnetically coupled through thin conductive layers, such as Ru. In such a SAF, multiple magnetic layers interleaved with a thin layer of Ru may be used. In another embodiment, the coupling across the Ru layers can be ferromagnetic. Although a single reference layer 16 or 26 and single tunneling barrier 18 or 24 might be used, the dual MTJ 10 may have the benefits of enhance spin torque if the reference layer 16 and 26 are fixed in in the dual state (magnetic moments 17 and 25 of reference layers 16 and 26 antiparallel). However, a dual MTJ 10 in the dual state may have reduced magnetoresistance. In contrast, if the reference layers 16 and 26 are fixed in the antidual state (magnetic moments 17 and 25 of reference layers 16 and 26 parallel) the dual MTJ 10 may have enhanced magnetoresistance. Moreover, in the antidual configuration, the spin-transfer torque contributions from two reference layers 16 and 26 counteract each other. As a result, in the antidual state, the amplitude of the spin-transfer torque on the free layer may be substantially reduced comparatively to that for a dual state or even that for a similar cell with a single barrier. Therefore, the read error rate, which is a probability of an unintentionally switching the free layer during the read operation, may be significantly reduced. This may allow significantly increasing the sensing margin (which is the difference between the minimum read current acceptable by the sense amplifier and the current resulting in intolerable read errors), enabling read currents to be close to the write currents in the dual configuration. This may also allow relax the requirements on the MTJ cell parameters, particularly on the thermal stability of the MTJ cell, since the read error rates depend on the cell thermal stability. However, this also means that spin transfer based switching may require a larger write current.
The conventional free layer 20 has a changeable magnetization 21. Although depicted as a simple layer, the conventional free layer 20 may also include multiple layers. For example, the conventional free layer 20 may be a synthetic layer including magnetic layers antiferromagnetically or ferromagnetically coupled through thin conductive layers, such as Ru. Although shown as in-plane, the magnetization 21 of the conventional free layer 20 may have a perpendicular anisotropy. Similarly, the magnetization 17 of the conventional pinned layer 16 may also be perpendicular to the plane.
To switch the magnetization 21 of the conventional free layer 20, a current is driven perpendicular to plane (in the z-direction). The current carriers are spin polarized and exert a torque on the magnetization 21 of the conventional free layer. In the conventional dual MTJ 10, the spin torque from the reference layers 16 and 24 would be additive as these layers are in the anti-dual state (magnetic moments 17 and 25 antiparallel). The spin transfer torque on the magnetic moment 21 of the conventional free layer 20 is initially small when the magnetic moment 21 is parallel to the easy axis (the stable state). As such, the stable state of the magnetic moment 21 also corresponds to a stagnation point in switching. Due to thermal fluctuations, the magnetic moment 21 may rotate from alignment with the easy axis of the conventional free layer 20. The spin transfer torque may then act to increasing effect, and the magnetic moment of the free layer 20 switched. When a sufficient current is driven from the top contact 30 to the bottom contact 11, the magnetization 21 of the conventional free layer 20 may switch to be parallel to the magnetization 17 of the conventional reference layer 16. When a sufficient current is driven from the bottom contact 11 to the top contact 30, the magnetization 21 of the free layer may switch to be antiparallel to that of the reference layer 16. The differences in magnetic configurations correspond to different magnetoresistances and thus different logical states (e.g. a logical “0” and a logical “1”) of the conventional MTJ 10.
When used in STT-RAM applications, the free layer 20 of the conventional MTJ 10 is desired to be switched at a relatively low current in order to prevent damage to the conventional magnetic junction 10, reduce the size of the transistor which supplies this current (not shown) and reduce energy consumption for the memory operation. In addition, a short current pulse is desired to be used in programming the conventional magnetic element 10 at higher data rates. For example, current pulses on the order of 5-30 ns or less are desired to allow the magnetization of the conventional free layer 20 to switch faster.
Although the conventional dual MTJ 10 may be written using spin transfer and used in an STT-RAM, there are drawbacks. For example, the write error rates may be higher than desired for memories having an acceptable pulse width. The write error rate (WER) is the probability that a cell (i.e. the magnetization 21 of free layer 20 of the conventional magnetic junction) is not switched when subjected to a current that is at least equal to the typical switching current. The WER is desired to be 10−9 or less. However, very high currents can be required to achieve switching of the conventional free layer 20 at this WER value. In addition, it has been determined that the WER may be challenging to improve for shorter write current pulses. For example, FIG. 2 is a graph 50 depicts trends in WERs for pulses of different widths. Note that actual data are not plotted in the graph 50. Instead, the graph 50 is meant to indicate trends. The pulse width, from longest to shortest, is for curves 52, 54, 56, and 58. As can be seen in the graph 50, for higher pulse widths, the WER versus voltage applied to the junction 10 has a higher slope. Thus, application of a higher voltage for the same pulse width may bring about a significant reduction in the WER. However, as the pulse widths shorten in curves 54, 56, and 58, the slope of the curves 54, 56, and 58 decreases. For a decreasing pulse width, an increase in voltage and/or current is less likely to bring about a reduction in the WER. At sufficiently short pulses, even high voltages/currents do not result in a lower error rate. Consequently, memories employing the conventional MTJ 10 may have unacceptably high WER that may not be cured by an increase in voltage. Further, to obtain such a high spin transfer torque the reference layers 16 and 26 have their magnetic moments 17 and 25 in the antidual state (fixed in opposite directions). When in this state, there is a cancellation of magnetoresistance during a read operation, which lowers the read signal. Such a reduction in signal is undesirable.
Accordingly, what is needed is a method and system that may improve the performance of the spin transfer torque based memories. The method and system described herein address such a need.